In co-rotating twin-shaft extruders equipped with double-flighted intermeshing screws, the total material flow is divided in the process space, i.e. the cavity in the extruder housing, into three parallel product flows extending along the screws and the inside wall of the extruder housing. The ratio of inside housing circumference to process space length is about 1:1.5 to 1:5. In contrast, in an extruder with e.g. twelve co-rotating parallel shafts disposed along a circle, i.e. forming a ring, the material flow is distributed over twelve axis-parallel product flows, and the ratio of inside housing circumference to process space length is between 3:1 and 1:1.5. Such an extruder is characterized by a substantially greater circumference based on its length compared to a twin-shaft extruder. Since four times as many individual flows of equal size must be guided parallel to each other within the process space, both the process engineering conditions and the possibilities of advantageously exploiting them are greater in an extruder having at least three co-rotating shafts.
In screws closely intermeshing on the whole circumference, the axial profile of each flight is determined by three circles corresponding to the outside diameter of the screw, the screw core diameter and the center distance of the screws (cf. EP-B-0 002 131).
In an extruder having plural co-rotating parallel shafts, the total process space in the extruder housing is divided into an outside and an inside process space. The two spaces are separated only by a thin, dynamic gap which is required for compensating production tolerances so that the screw shafts are freely rotatable without touching each other. However, the closely intermeshing screws prevent material from passing from one process space to the other, at least in the case of solid or highly viscous materials. When such materials are supplied, only the outside process space is therefore filled through the supply port while the inside process space remains unused in the area of the supply port. Due to this feed restriction, only a fraction of the possible throughput is realized by this extruder.
According to EP-B-0 788 867, the supply port of the extruder in the ring of shafts therefore has at least one opening which is formed for example by replacing one screw element by a cylindrical sheath. This improves the exchange of material between the outside and inside process spaces but reduces the conveying properties in the area of the supply port.
The problem of the invention is to provide an extruder having at least three co-rotating shafts parallel to the extruder axis with maximum throughput.